The nineteen-forties gave birth to information theory. Claude Shannon and co-workers showed that the bandwidth of a transmission channel and its signal-to-noise ratio are not the only parameters to be optimized in order to transmit data without errors but that coding can be used to improve the bit-error ratio in transmission. Efficient coding algorithms became the goal of many a communication systems designer in the last eighty years, and this has resulted in a dramatic increase in bandwidth efficiency of modulation used in microwave communications today: from less than 1 bit/s/Hz to 30 bits/s/Hz for 4G cellular technology. In order to maintain transmission at a high aggregate data rate over appreciable distance, the signal-to-noise ratio has to remain large. The Shannon-Hartley theorem states that the maximum information transfer rate (channel capacity) in the presence of noise is given by
                    C        =                  B          ⁢                                          ⁢                                    log              2                        (                          1              +                              S                N                                      )                                              (        1        )            where S/N is the signal-noise ratio and B is the channel analog bandwidth. It is clear that no matter how good the coding is, for a given channel bandwidth, the channel capacity depends on the signal to noise ratio. This ratio is limited by the maximum practical power a transmitter can emit on the high end of the signal power and the receiver sensitivity and distance of propagation on the low end. The receiver sensitivity is determined by input device noise figure and the received in-band noise which depends on the bandwidth of the receiver. The end points have generally been set by government bodies and adjusted with the advent of technology. There are numerous applications where the upper power limit is not regulated, but rather determined by required distance of propagation and required bandwidth or often by practicality. This is the case with systems intended to emit signals over large distances and with aerospace applications. In these applications, amplifier output powers can range from kilowatts to hundreds of kilowatts. The design of RF and microwave transmitters delivering such powers presents a challenge.
Solid-state amplifiers use transistors of achieve high RF output powers, the device of choice is the heterojunction field-effect transistor (HFET), which is often referred to as high-electron-mobility transistor (HEMT). It would be ideal if one could manufacture a 10-kW amplifier with only one chip. This is not possible because of a number of practical limitations. Transistors are semiconductor devices that come built on semiconductor chips which are generally small—their sizes measure in millimeters and centimeters at most. Besides the size, which limits the thermal dissipation, there are other reasons: the maximum RF power that can be emitted from a field-effect transistor (FET) is determined by its breakdown voltage VBR and its current carrying capability. For a given technology and epilayer design, the breakdown voltage reduces with the operation frequency and hence the only way to arbitrarily increase the output power (the product of voltage and current) is to increase the transistor current. The drain-to-source current density [A/mm], on the other hand, is determined by epilayer design and technology. With the current density (A/mm) and operation voltage set, the only free parameter that can be used to increase the transistor output power is the transistor periphery. The transistor periphery is a measure of effective gate width perpendicular to the current flow.
The following is a discussion of the options a designer has in designing a high power solid-state amplifier using transistors, specifically, high-electron mobility transistors (HEMTs). Transistor is an electronic device built on top of semiconductor layers, the body of a transistor includes both the semiconductor layer design and the top surface geometry which includes metal layers, contacts and passivation. When one semiconductor chip contains one transistor, it is often referred to as a discrete component and the chip most often have a rectangular shape elongated in direction that is perpendicular to the flow of the RF signal and hence are often referred to as transistor bars. The width of the transistor on the chip determines the transistor bar length. In this specification, the longer dimension of the transistor chip bar is referred to as the transistor bar width. When semiconductor chips contain multiple electronic devices on the same chip, such as passive matching elements, dividers, driver circuits with more transistors in addition to one or more high power RF transistors, this part is referred to as a monolithically integrated microwave circuit (MMIC) with an amplifier. The MMIC chip width is the dimension of the MMIC chip along a direction that is perpendicular to the direction of the RF signal across the output power transistor in the MMIC. The shape of the output transistor and design rules discussed below are the same for transistor chip bars and amplifier MMIC chips.
In practical RF power transistors, the periphery is broken into an integer number of segments. The number of segments is often referred to as the number of gate fingers (F) and the length of each segment is referred to as unit gate width (W). The periphery of a transistor is then defined by P=F·W. High power RF transistors in standalone form or in a microwave monolithically integrated circuit (MMIC) are built in an elongated design in which the unit-gate fingers are disposed in a periodic fashion with their direction perpendicular to the direction of the RF signal as it is well known in the art. The RF power direction is parallel to the gate fingers and inasmuch as their length presents a phase delay of the RF signal, the desired frequency of operation sets the maximum gate width (W˜100 μm for X-band or W˜50 μm for Ka-band). This leaves only the number of fingers F as the parameter determining the RF transistor power. The width of the RF transistor pattern or chip is determined by the average gate-finger pitch S and the number of fingers F. The width of the transistor on any chip is at least F·S. The pitch is taken as an average as some manufacturers tend to group the gates in pairs so that there technically exist two gate to gate separations.
Each of the unit gate fingers is a heat source, so that the minimum pitch between the gate fingers is limited by electrical conductivity of the source and drain contacts and by thermal crosstalk, which in turn is primarily determined by the thermal conductivity of the substrate on which the transistor is made. The average pitch is the average distance between gates in the direction that is perpendicular to the gates. For commercially available AlGaN/GaN HEMTs on silicon carbide and silicon, the average pitch values range from around 35 um to 60 um. Standalone RF power transistor-bar chips and RF power amplifiers that use transistors integrated into MMICs are commercially available from companies like Qorvo of Greensboro, N.C. and Cree of Research Triangle Park, N.C. The typical substrates for present-day high-power RF transistors are GaAs for GaAs p-HEMTs, silicon for laterally-diffused MOS (LDMOS) transistors, and SiC for AlGaN/GaN high-electron mobility transistors. Silicon carbide is the substrate with highest thermal conductivity in commercial use today. Consequently, given the technology and cross-sectional design, the lateral dimension of the transistor design is the only design parameter that determines the transistor RF output power and hence building transmitters of increasing RF power amounts to managing wide transistor-chip bars or wide MMIC chips, namely, large chip widths. This fact is well known in the industry and examples of designs that make use of the knowledge can be found in publicly available books, such as: Steve Marsh, Practical MMIC Design, available from Artech House in Boston, USA.
There is a practical limit to how wide transistor-chip bars or how wide MMIC chips can be manufactured and this limit is related to the mechanical stability of the chips and process yield. This maximum width is of the order of several millimeters as can be seen in the commercial offering of the abovementioned providers. The challenge in obtaining RF transmitter modules with power greater than several tens of Watts lies with packaging several transistor-chip bars into multichip modules. A simplified top-view illustration of an exemplary packaged single-stage RF amplifier with four transistor-bar chips is illustrated in FIG. 1 (Prior Art). The packaged amplifier 100 comprises a flange 101 with an elevated rim 102 that encloses the package interior 105, an RF input lead 103 and an RF output lead 104. The package interior will ultimately be closed by soldering a lid (not shown to allow the interior to be seen) to the rim 102. The RF signal is directed from the input RF lead 103 towards the output RF lead 104. The RF input lead that protrudes into the package interior first delivers the RF signal to a microstrip RF power divider chip 107, whose function is to convert the external transmission impedance to the input impedance of the transistor on each transistor-bar chip 106 and to divide the input RF power into a multiplicity of identical transistor-bar chips 106. Packaged amplifier 100 is shown as having four identical transistor-bar chips 106 with power divider microstrip pattern 108 disposed on chip 107 dividing the input power into four ports attached to each bar using wire bonding 109. The output signal from each of the transistor-bar chips is combined using power combiner chip 110 and coupled to the output RF lead 104. Microstrip pattern 111 disposed on output power combiner chip 110 serves to combine the power from each of the transistor-bar chips 106 and transforms the impedance of the transistor manufactured on the transistor-bar chip 106 to the impedance of the external transmission lines (the impedance transforming pattern is not shown).
In such a highly parallel design, as is shown in the packaged amplifier 100, the three most important design challenges are (1) signal path matching, (ii) amplifier stability, and (iii) thermal limitations. Signal-path matching (i) amounts to ensuring that the RF path through each of the amplifiers from the input 103 to the output 104 is identical so that the signals can constructively add at the output power combiner 111, otherwise some of the RF power output from one of the transistor-bar chips will be dissipated due to destructive interference with signals from a neighboring transistor-bar chip. Amplifier stability (ii) is strongly dependent on the electromagnetic crosstalk between the transistor-bar chips and lateral electromagnetic resonances in the package. Both of these have to be minimized and eliminated in order to maintain the amplifier stability and prevent any power sinks due to resonances or oscillation. Amplifier stability becomes progressively more difficult to manage as the amplifier width 112, determined by the number of transistor-bar chips 106 in parallel, increases and becomes comparable to the wavelength at the operating frequency. The signal path management and amplifier stability place a limit to how large the package can be and how many parallel transistor-bar chips can be placed within it. Thermal limitations (iii) limit the maximum amount of power one can obtain from a package and hence how small can the package be for a given amount of power.
To go beyond these limits, one prior art approach is to combine amplifier packages (or modules) into amplifier systems in which the input signal is first divided among a multiplicity of separate amplifier packages and then the output of each of the amplifier packages is combined to provide one output. Inasmuch as the dimensions are now significantly larger than the dividers and combiners in the package, one has the possibility of using efficient power combiners made with waveguides, for example. Waveguide combiners generally tend to show lower combining loss than microstrip combiners. The advantage of this modular approach is that the physical separation between the amplifier packages results in dramatically reduced electromagnetic crosstalk and reduced thermal crosstalk, and hence higher amplifier stability.
FIG. 2 shows a block diagram 200 of a high-power RF amplifier using a multiplicity of packaged amplifier modules denoted 201. The figure shows eight modules 201, but the module number m is arbitrary. The RF signal with power PIN is brought to input port 202 and split into m equal power channels using RF power divider section 203. Power divider section 203 uses a binary tree of 1×2 Wilkinson power dividers 204 and hence there is an advantage of making m=2n, where n is an integer. There are other ways of splitting the power, for example, using radial power combiners, hybrid junctions or power dividers that split one input into an odd number of outputs, as is well known in the art. The RF input signal is split into m channels, all ideally having the same amplitude and phase before they are amplified in each amplifier 201 as power P1. The signal with power P1 is amplifier with each amplifier 201 giving output power P2. The output P2 of each amplifier 201 is then combined using power combining section 205, which is also shown using 1×2 Wilkinson power combiners. Any RF losses in the input power divider 203 and output power combiner 205 reduce the amplifier efficiency. Output power combiner 205 tends to be designed for higher power handling and lower loss than input-stage power divider section 203, because more power may potentially be lost on the output combiner than on the input divider. The RF combined output is then fed to external port 206. The power dividers and combiners tend to be large and heavy, as waveguides generally exhibit lower loss than microstrip technology used for the input stage and inside the amplifier package.
An example of a commercial X-band multiple kW amplifier system 300 is shown in FIG. 3, including four RF power amplifier modules 301, 302, 303, and 304 separated with heavy metal heat conducting elements 308, and one waveguide output 306. The output from each of the amplifier modules is combined using a waveguide combiner 305 into waveguide output 306. Only the output power combiners are shown. The thermal management of this exemplary high-power RF amplifier system 300 includes liquid cooling using copper pipes 307 of which only the rounded edges are visible in this figure.
RF splitters, dividers, and combiners (if operated in reverse) are terms used for passive components that can be used to split RF power coming in from one port into multiple ports, most often two ports, but the output port number may be in some applications rather large. The terms splitters or dividers refer to RF signal division, while combiners refer to RF signal combining even though splitters, dividers and combiners may physically be identical due to reciprocity. The terms splitter, divider and combiner are often used interchangeably, although care should be taken regarding whether all of the ports are intended to be matched to the transmission lines and also whether the matching is adjusted to any specific line impedance. Furthermore, the splitters/combiners may be designed for more than one function: splitting the power and matching impedance at the same time. We shall refer to all of them for the remainder of this disclosure as combiners. There are resistive and hybrid combiners, and they differ in their insertion loss. Resistive combiners are generally broadband, but use resistors and are not used in the output stages of power amplifiers. Hybrid combiners use impedance transformation to perform RF signal combining or splitting, and are ideally lossless. The insertion loss of a splitter is a figure-of-merit of the quality of a splitter and it generally includes the loss of RF power due to the division of RF power to other ports, the part that is not actually lost as it has to be directed to another port, and the RF power that is lost due to radiation at the junction or transformation loss. The insertion loss (IL) of a power splitter/combiner is defined as IL=−10·log10[Pm/P0], where P0 is the input RF power at the common port, and Pm is the output RF power at one of the m output ports. RF combiners come in a large variety: on chips and inside packages the combiners are generally planar microwave circuits which use impedance transformation to accomplish the splitting and combining of RF signals without too much loss. The power combining outside of the package may be realized using microstrip traces, and/or using coaxial combiners and high power waveguide combiners. Waveguide combiners (like the example shown in FIG. 2) are large, but also exhibit low loss at high frequency.
It is clear that reaching high power RF output in amplifier systems requires massive microwave hardware and sophisticated thermal management. The higher the frequency, the lower the contributed power per amplifier, especially when InP or GaAs based materials are used, because of their low breakdown voltage. It is also clear that the size of the completed amplifier system at kW levels is directly determined by the size of the amplifier chip, as was discussed above, and that given the maximum size and weight constraint, the most efficient way to increase the output power of these modules is to improve the output power P2 of each of the chips situated in the amplifier 201 in FIG. 2. (This is equivalent to increasing the maximum output power of each of the transistor chips 106 in FIG. 1.) By increasing the power output P2 of an individual transistor chip 106 one will improve the output power POUT of the overall RF amplifier module.
RF combiners may be realized as a microstrip pattern on an amplifier chip, integrated into an MMIC chip or provided on a dielectric chip to be placed next to the MMIC within an RF amplifier package (example shown with 108 in FIG. 1). RF combiners may also be realized as an electromagnetic waveguide combiner (see, for example, element 305 in FIG. 3) or as coaxial RF combiners.
It is hence clear that there is a need in the industry for high power RF amplifiers that are smaller and lighter for given power, and hence have extended reach, as improving the signal-to-noise ratio delivers the possibility of error-free communication over a larger distance.